JPH0377070A - Current detecting device - Google Patents

Abstract

PURPOSE:To obtain tolerance to a common mode noise and to stably measure a DC current by supplying a current to be measured to a detection resistance and generating a DC voltage, converting the DC voltage into an AC signal and amplifying it, and converting the signal into a DC signal. CONSTITUTION:The current IL from a power source 1 flows through the detec tion resistor 2 and an electric load 3 to generate the DC voltage Vs across the resistor 2. When a switching circuit 9 switches in synchronism with a pulse signal ex applied to a control terminal 93, the AC signal em having an amplitude proportional to the voltage Vs is generated at the output side of the circuit 9. When the signal em is applied to the transformer 10, a signal es from which a DC component is removed appears at the secondary side of the transformer 10. Then the signal es is passed through an AC amplifier 11 to become an AC signal eo, which is inputted to an AC/DC converting circuit 12. The circuit 12 generates an impulsive signal ep having a mean value proportional to the amplitude of the signal eo and the signal is passed through a smoothing circuit 13 to remove an AC component from the signal ep, thereby obtaining the DC signal VOUT which is proportional to the mean value of the signal ep.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a circuit for accurately and stably measuring direct current. The present invention relates to a current detection device that is highly effective.

[Conventional technology]

FIG. 6 is a diagram showing the most common current detection method. In Fig. 6, l is a power source for supplying current IL to electric load 3, 2 is a current detection resistor, and its resistance value is R8.
shall be. Reference numeral 4 denotes an instrument cylinder amplifier, which can provide an output voltage v ouy at an output terminal 43 that is proportional to the voltage between its two input terminals 41 and 42.

In FIG. 6, since VI=11R3, if the amplification degree of instrumentation amplifier 4 is A, then yo
The current IL flowing through the electrical load 3 can be measured with ua=A.■,.R8.

Generally, the voltage drop across the detection resistor 2 is undesirable, so the resistance value R4 is set as small as possible. For example, if the maximum measured current is 100A and the voltage drop across detection resistor 2 is 10mV, then R8 is 0.
．． 1 d. At this time, if IL-10A is used, V
r -1 mV, which is a fairly small value for a voltage level handled by the instrumentation amplifier 4. Such current detectors are also used for measuring the output current of motor vehicles, where the ambient temperature of the detector varies from -40°C to +100'C. Furthermore, in automobiles, the car body is used as an electrical ground, so current detection must be measured on the tfA side (usually the positive side), so the detection resistor 2
The potential of is equal to the voltage of the vehicle battery. Due to the influence of fluctuations in various on-vehicle electrical loads, the power supply for automobiles is normally 1.
There are fluctuations that exceed the voltage. Therefore, the instrumentation amplifier 4 must amplify a minute voltage such as 1 mV under common mode noise of 1 volt or more.

Although it is not technically impossible to obtain an instrumentation amplifier that can stably amplify voltages of 1 mV or less under such environmental conditions, it is difficult to do so when considering the cost involved. , not realistic.

FIG. 7 is a diagram showing the principle of a method used for the purpose of solving the above-mentioned difficulties, and this method was first published in Japanese Unexamined Patent Publication No. 6
3-. This method is described in detail in Japanese Patent Publication No. 228071, Japanese Unexamined Patent Publication No. 1-113674, and the like.

In FIG. 7, reference numeral 5 denotes a ferromagnetic ring having a slit-like notch in a part thereof, and a conducting wire 20 is disposed so as to pass through this. L flows. A Hall element 6 is disposed in the notch of the ferromagnetic ring 5 so as to measure the circumferential magnetic field of the notch of the ferromagnetic ring 5 . 7 is a power supply for supplying current (or voltage) to the Hall element 6; 8 is an amplifier for amplifying the output voltage of the Hall element 6; its output terminal 81;
The voltage obtained at V. Let it be U. Naturally, the magnitude of the magnetic field applied to the Hall element 6 is determined by the current 1.
Since it is proportional to, ■1 and V. The relationship between uT is given by the following equation.

■,, T=, It+VO-... (1) (1) In 2, K is a constant and is determined by the dimensions of the magnetic ring 5, the sensitivity of the Hall element 6, and the amplification degree of the amplifier 8. . vo is the offset voltage, the residual magnetism of the magnetic ring 5, the Hall element 6
is determined by the unbalanced voltage of the amplifier 8 and the offset voltage of the amplifier 8. The method shown in FIG. 7 is extremely useful because it measures the current after converting it into a magnetic field, so there is no need to worry about voltage drop due to the detection resistor 2 as in the method shown in FIG. ) K and ■ in the formula. There are many ways to maintain a stable and constant value. That is, among the elements that determine the constant K, the sensitivity of the Hall element 6 has a large variation in characteristics from component to component and is highly temperature dependent, and among the elements that determine the offset voltage V0, the sensitivity of the Hall element 6
The unbalanced voltage also varies widely from component to component and is temperature dependent. For this reason, in the t-current detector using the method shown in FIG. 7, some kind of adjustment is required in the manufacturing process to compensate for variations in sensitivity and unbalanced voltage of the Hall element 6, and temperature compensation is also required. There is a need. The temperature dependence (temperature characteristics) of the sensitivity and unbalanced voltage of the Hall element 6 will show the same tendency if the type (model name) of the Hall element 6 is the same, but it is guaranteed that they have exactly the same temperature coefficient. Therefore, even if temperature compensation is performed, temperature dependence cannot be completely eliminated.

[Problem to be solved by the invention]

As mentioned above, in the method of converting the current to be measured into voltage using a detection resistor, it is necessary to amplify the minute DC voltage in the presence of large common mode noise, and it is necessary to amplify the small DC voltage in the presence of large common mode noise. The method of measuring with a Hall element requires adjustment to reduce the effects of variations in the sensitivity of the Hall element and unbalanced voltage, and also requires compensation for the temperature characteristics of the above two parameters, making complete compensation difficult. There were issues such as:

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to obtain a current detection device that is resistant to common mode noise, does not require adjustment, and is stable against temperature changes.

[Means to solve the problem]

The current detection device according to the present invention converts current into voltage using a detection resistor with a sufficiently moderate resistance value so that voltage drop does not become a problem, and further converts this voltage into alternating current using a switching circuit, and then converts the current into alternating current via a transformer. The AC is amplified by leading it to an amplifier,
By converting the signal after AC amplification into DC again, a DC signal proportional to the magnitude of the current to be measured is obtained.

[For production]

The current detection device of the present invention converts a DC voltage proportional to the current to be measured into an AC voltage having an amplitude proportional to this voltage, and uses an AC amplifier that is essentially unaffected by temperature and power supply voltage fluctuations to detect a sufficiently large level. After amplification, it is converted to DC using an AC/DC converter circuit.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
In the figure, 1, 2.3 are a DC power supply, a detection resistor, and an electric load that are connected in series to form one closed loop as in Figure 6, and the detection resistor is connected to the non-grounded side of the power supply l. The current ■ flowing through 2 is the current to be measured. 9 is a switching circuit for generating an alternating current signal, and an input terminal 91 is connected to the high potential side of the detection resistor 2, and a common terminal 92 is connected to the low potential side thereof. 10 is a transformer, and input terminals 100, 101 on the next side are output terminals 94.1 of the switching circuit 9. It is connected to the common terminal 92. An output terminal 103 at one end of the secondary side of the transformer 10 is grounded, and an output terminal 102 at the other end is connected to an input terminal 110 of the AC amplifier 11. 12 is an AC/DC conversion circuit, and the input terminal 121 is the output terminal 111 of the AC amplifier 11.
A common terminal 122 and an output terminal 12 connected to and grounded.
It has 3. 13 is a smoothing circuit whose input terminal 131 is connected to the output terminal 123 of the AC/DC conversion circuit 12;
It has a grounded common terminal 132 and an output terminal 133. 14 is an oscillator having an output terminal 141; 15 is a drive circuit in which an input terminal 151 is connected to the output terminal 141;
Its output terminal 152 is connected to the control input terminal 93 of the switching circuit 9.

Next, the operation of one embodiment of the present invention will be explained with reference to FIG. A current IL flows from the power source I to the detection resistor 2 and the electric load 3, and the current IL flowing to the detection resistor 2 is the current to be measured. The switching circuit 9 has an input terminal 91 and a common terminal 92.
The DC voltage (■) applied between both ends of the detection resistor 2 is switched synchronously with the pulse signal applied to the control input terminal 93, and V! is applied between the output terminal 94 and the common terminal 92. An alternating current signal (pulse train) ea having an amplitude proportional to is generated. This pulse train e1 is applied between the input terminals 100 and 101 on the primary side of the transformer 10, and is guided between the output terminals 102 and 103 on the secondary side of the transformer lO with the DC component removed, and this signal is e , this signal e is applied to the input terminal 110 of the AC amplifier 11, and its output terminal 11
Obtain the output eo amplified to 1. The AC/DC conversion circuit 12 generates a pulsating current signal e having an average value proportional to the amplitude of the AC signal (pulse train) eo from the AC amplifier 11 applied between the common terminal 122 and the input terminal 121.

The 6 smoothing circuit 13 that obtains the output terminal 123 has the input terminal 1
The AC component is removed from the pulsating current signal e from the AC/DC conversion circuit 12 applied to 31, and the human power signal (pulsating current signal) ep is
The output terminal 133 outputs a DC signal vout proportional to the average value of
get to.

On the other hand, the oscillator 14 applies a pulse signal with a repetition frequency f to the input terminal 151 of the drive circuit 15 via the output terminal 141. The drive circuit 15 generates at an output terminal 152 a pulse signal eX having a repetition frequency f and a DC level and amplitude necessary for operating the switching circuit 9 as a switch, and applies it to the control input terminal 93 of the switching circuit 9. .

For the case where a current l is flowing through the electric load 3, a more detailed explanation will be given with reference to the signal waveforms of each part. FIGS. 2(a) to 2(f) show signal waveforms of each part.

(a) is the voltage V across the detection resistor 2, which is also an input signal to the switching circuit 9, and its value is
1.R1. (b) is the output voltage of the switching circuit 9, that is, the input signal e of the transformer 1o, which is a pulse train with a repetition frequency f and an amplitude of +'lt'Rs.

Here, KI is a constant determined from the configuration of the switching circuit 9, the impedance of the transformer LO, and the human power impedance of the AC amplifier 11. (C) is transformer 10
The output voltage of the AC amplifier]1 is the input signal e, and
This is a pulse train with an amplitude (peak-to-peak) of 1, 2, and R8. @ is the output voltage of the AC amplifier 11, that is, the input signal eO of the AC/DC conversion circuit I2, and its amplitude is G'K+'IL'Rs. Here, G is the amplification degree of the AC amplifier 11. (e) is AC/
The output of the DC conversion circuit 12, that is, the input of the smoothing circuit 13 (
The g number is e-, the low level is zero and the high level is 2.
・G-, ・IL, R, pulse train. Here! is a constant related to the efficiency of the AC/DC conversion circuit 12. (f) is the output signal v out of the smoothing circuit 13, and its value is: ・R8・G'K 11 L ,R
It is s.

K is a constant given as the product of the pulse rate (duty factor) of the signal e2 and the amplification degree of the smoothing circuit 13, and when the duty factor of eF is 50% and the smoothing circuit 13 is a simple CR low-pass filter. If l=0
．． It becomes 5.

To summarize the above, the output voltage VOL+T is VOIIT=
K+・Kz'Kx'G-Rs・r L= It'
Rx ''' (2) where, in R,=,
, ・K, ・G'Rs As is clear from the explanation of 0 or more, each constant that determines the conversion parameter R,
, to,, to 3. G.

R8 can be uniquely determined at the circuit design stage, and is subject to temperature changes,
Errors due to component variations can be kept within the range specified as design specifications.

FIG. 3 is an example of a switching circuit 9, where the input terminal 9
Two resistors 95 connected in series between 1 and the output terminal 94
．． The FET 97 has its drain and source connected between the common terminal 92 and the connection point between the FET 96 and the resistor 95 and 96, and the gate of which is connected to one terminal of a capacitor 99. A resistor 98 connected between the gate terminal of the FET 97 and the common terminal 92 is for fixing the DC potential of the gate. The other terminal of capacitor 99 is connected to output terminal 152 of drive circuit 15 . When a positive pulse is applied to the gate of the FET 97 via the capacitor 99, the FET 97 is turned on and the drain and source are almost short-circuited, and the output voltage e becomes zero. When a negative pulse is applied to the gate of the FET 97 via the capacitor 99, the FET 97 is turned off and voltages TL and R are generated at the output terminal 94 as described above.

FIG. 4 shows another embodiment of the switching circuit 9, in which a PNP transistor 971 is used as the switching element. This method is suitable for automotive applications. In other words, in the case of an automobile, all electric loads are operated by a single DC TFI (battery) 1, so the potential of the common terminal 92 of the switching circuit 9 and the potential of the power supply for operating the current detection device are approximately the same. Same.

Therefore, in FIG. 4, when the voltage applied to the control input terminal 93 becomes low level by the drive circuit 15, the transistor 971 is turned on, and the control input terminal 93
When the transistor 971 becomes open, the transistor 971 is turned off and operates in the same manner as in FIG. In the case of the switching circuit shown in FIG. 4, the output circuit (not shown) connected to the output terminal 152 of the drive circuit 15 is suitably an oven collector type NPN transistor.

FIG. 5 shows a specific embodiment of the AC/DC conversion circuit 12, which is known as a phase detection circuit. Fifth
In the figure, a series circuit of a resistor 124 and a capacitor 125 is connected between an input terminal 121 and an output terminal 123, and the drain and source of an FET 126 are connected between the output terminal 123 and ground. Unlike FIG. 1, the circuit shown in FIG. 5 has a control input terminal 127, which is connected to the gate of FET 126 and to the output terminal 141 of oscillator 14. .

According to the circuit shown in FIG. 5, the DC component of the signal obtained at the input terminal 121 is completely removed by the capacitor 125, and the signal applied to the control input terminal 127
26 turns on and off synchronously with the signal frequency, noise components are effectively removed from the average value of the signal obtained at the output terminal 123.

This is a major feature of the phase detection circuit.

[Effects of the Invention] As described above, according to the present invention, a minute DC voltage is generated by passing a current to be measured through a detection resistor, and this DC voltage is converted into an AC signal by a switching circuit, and is sent to an AC amplifier via a transformer. The structure is configured so that the output is obtained by converting the signal into a DC signal after amplifying it, so it is resistant to common mode noise, is not affected by temperature changes or component variations, and does not cause excessive voltage drop in the circuit under test. This has the effect of being able to stably measure direct current without causing any damage.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
3 and 4 are circuit diagrams showing a specific configuration example of a switching circuit, and FIG. 5 is a circuit diagram showing a specific configuration example of an AC/DC conversion circuit, 6th
7 are diagrams showing circuits for explaining the prior art. In the figure, 2... detection resistor, 9... switching circuit,
1(]...Transformer, 11...AC amplifier, 12.
・・AC/DC conversion circuit, 13・・Smoothing circuit, 14・
...Oscillation circuit, 15...Drive circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] A switching circuit that takes a detection resistor through which a current to be measured flows and a voltage generated across the detection resistor as one input, and switches the input signal to convert it into a pulse signal according to a signal applied to a control input terminal; A transformer whose input is the output of the circuit, an AC amplifier whose input is the output of the transformer, an AC/DC converter circuit which converts the output of the AC amplifier into DC, and an input terminal which is the output of the AC/DC converter circuit. A current detection device comprising: a smoothing circuit connected to a terminal; and a drive circuit that receives a signal from an oscillator and applies a pulse signal to a control input terminal of the switching circuit.